The invention is directed to the field of electronic reproduction technology and concerns a method for exposure calibration of recording devices for the pixel-by-pixel and line-by-line exposure of rastered surfaces such as pictures or color separations on a recording material with at least one exposure beam generated in an exposure unit. The invention is also directed to an apparatus for the implementation of the method. A recording device of this type is also called an exposer, printer, or recorder.
For example, the recording device can be a color separation recorder for producing rastered color separations for multicolor printing with which rastered color separations "yellow", "cyan", "magenta" and "black" are exposed point-by-point and line-by-line. Disturbing Moire phenomena can occur given superimposed printing of the color separations to produce a multicolor print. In order to avoid these Moire phenomena, the individual color separations are exposed with rasters having different screen angles and/or screen widths. For example, the color separation "yellow" is exposed with a screen angle of 0.degree., the color separation "cyan" is exposed with a screen angle of +15.degree., the color separation "magenta" is exposed with a screen angle of -15.degree., and the color separation "black" is exposed with a screen angle of 45.degree..
A raster is composed of a plurality of periodically repeating raster meshes or raster cells in which raster points (raster spots) having different sizes are generated dependent on the gray scale values (tonal values) to be reproduced. Every raster point within a raster mesh is composed of pixels or picture elements that are recorded with the exposure beam. The respective ratio between the area of the raster mesh and the area of the raster point recorded in the raster mesh, referred to as degree of surface coverage, defines the reproduced gray scale value.
In such a color separation recorder, the picture signal values that represent the gray scale values to be recorded are supplied to a raster generator wherein the picture signal values are converted into corresponding control signal values for the exposure unit according to a raster function which is also referred to as a spot function. The control signal values switch the exposure beam on and off during a point-by-point and line-by-line relative motion between exposure beam and recording material, and thus determine what pixels are exposed or not as parts of the raster points on the recording material. The raster function thereby defines the size of the raster points dependent on the gray scale values to be recorded and also defines the shape of the raster point. In most cases, the relative motion between exposure beam and recording material occurs on the basis of a continuous or step-by-step conveying of the recording material in a conveying direction that is directed perpendicular to the line direction. For example, the exposure beam is a laser beam that is switched on and off with a modulator input with control signal values. The exposure unit can be designed such that it generates one exposure beam or a plurality of exposure beams lying side-by-side.
For example, DE-A-28 27 596 (U.S. Pat. No. 4,499,489) discloses a rastering method for recording color separations with rasters having arbitrary screen angles and/or screen widths.
When recording rastered pictures and color separations, the real raster point sizes or gray scale values (actual gray scale values) in fact generated on the recording material deviate from the desired, nominal gray scale values (rated gray scale values), since every pixel and thus every raster point is recorded more or less enlarged. The cause for the enlargement of the pixels is a halo that forms around every pixel and that is dependent on the intensity of the exposure beam. The difference between the gray scale values that are in fact generated and the nominal gray scale values is referred to as an increase in optical density, or is also referred to as increase in gray scale value.
Since an increase in optical density leads to disturbing changes in the tonal values of the reproduction compared to the original, the increase in optical density is compensated in practice in that a correction curve, also referred to as a transfer curve, is calculated by an exposure calibration in a calibration phase preceding the actual exposure, and the picture signal values that represent the nominal gray scale values are corrected according to the transfer curve during the exposure such that the gray scale values recorded in fact correspond to the nominal gray scale values.
Various parameters such as type of recording device, resolution of the recording device, density setting of the recording material respectively employed and the raster parameters such as raster function, screen width (screen frequency) and screen angle respectively employed must be taken into consideration in an exposure calibration.
In a traditional exposure calibration, a new, current transfer curve must be calculated over the entire calibration process, given modification of only one of these parameters. This necessity proves especially disadvantageous with respect to the raster parameters, since these are frequently changed. Given density measurements to be undertaken manually, involved and time-consuming calibration processes must be implemented in practice, the recording device thus not being available for useful exposures during these calibration processes.